The Flue is the pipe from the firebox to the smokebox that the gasses of combustion and fire travel through to heat the water. In the Ruby the firebox and the flue are all one pipe. In other steamers they can be separate areas (even multiple flues from the firebox through the boiler to the smokebox).
The Blast Pipe or Exhaust Stand is what the used steam exits through. On a gas fired loco (like the Ruby) the only use of this pipe is to try to simulate smoke up the stack by venting the steam there. On a coal, oil, or alcohol fueled loco the blast pipe helps to maintain a draft through the fire by causig a vacuum in the smokebox which pulls the fire gasses through the flue from the firebox.
